Molecular beam cesium getter



Feb. 21, 1967 GAN'SSEN ETAL 3,305,290

MOLECULAR BEAM CESIUM GETTER Filed March 5, 1963 FIG. I

LIQUID CESIUM I6 VAPOR PRESSURE OF Cs IO (mm of Hg) CESIUM ANTINIONIDE |O T 1 l IO 8 TEMPERTURE IN c.

"All MAGNET IO POLE PIECE SOURCE OVEN IINVENTORS ALEXANDER GANSSEN AMES GEORGE 1 ATTORNEYS United States Patent 3,305,290 MOLECULAR BEAM CESIUM GETTER Alexander Ganssen, Wakefield, and James George, Swampscott, Mass, assignors to National Company, Inc, Melrose, Mass, a corporation of Massachusetts Filed Mar. 5, 1963, Ser. No. 262,933 13 Claims. (Cl. 316-45) The present invention relates to a superior getter material for cesium and particularly concerns an improved molecular beam device containing this getter material. More particularly the invention relates to a metal getter for use with molecular beam devices which utilizes beams of atomic cesium for frequency control.

In molecular or atomic beam devices such as those described in US. Patent 2,972,115 to Zacharias et al., issued February 14, 1961, and US. Patent 2,994,836 by Holloway issued August 1, 1961, it has been the practice to employ suitable getter material in the evacuated beam tube. Getter material is commonly required to help maintain in the beam tube a pressure condition of 1X10 mm. or less. In molecular beam devices employing an alkali metal such as cesium as a beam source only a portion of the atomic cesium beam generated is used for frequency control, the remainder of the beam condensing on the internal walls of the beam tube between the cesium source oven and the detector. The cesium atoms striking and condensing on the internal walls of the beam tube generally constitute those atoms of undesired energy state, of unselected velocity distribution and those atoms striking the wall due to the solid angle effect by the diverging beam. The condensed cesium may be in the solid or liquid state depending upon the environmental temperature of the wall surface. Cesium has a melting point of 28.5 C. (833 F.) and therefore those condensed cesium atoms located near the cesium oven source are usually in the liquid state.

Molecular beam devices are commonly operated at ambient temperatures of about 20 to 30 C., e.g., 25 C. however, the oven source of cesium atoms is usually at an elevated temperature of from 50 to 150 C. such as 65 C., thereby producing a temperature gradient extending from the oven source outward usually as far as the A magnet. Liquid cesium has a vapor pressure of 10* mm. of mercury at a temperature of 25 C. (78 F.) and at 65 C. 140 F.), the typical operating temperature of liquid oven, the vapor pressure of cesium is approximately 50 times greater, i.e., mm. of mercury. The cesium atoms condensed on the beam tube wall and particularly those atoms condensed in areas having an elevated temperature that is, a temperature greater than the operating temperature of the molecular beam tube device, e.g., C., create a background vapor pressure greater than that desired. Those atoms condensed Within the temperature gradient area between the oven temperature and the operating temperature of the device such as between the cesium oven source and the A magnet or even more particularly between the collimator and the A magnet give rise to a particular serious problem. The increased vapor pressure of the condensed cesium acts as a residual gas and promotes the scattering of the cesium beam due to the enhanced possibility of molecular collision between the vapor from the condensed cesium and the cesium beam. This uncontrolled cesium atom flux gives rise to an observable background cesium detector current. This background current becomes serious as it approaches the order of magnitude of the cesium signal current and promotes a decreased signal to noise ratio.

To aid in suppressing residual gas in cesium background and scattering effects and thereby reduce background detector current it has been the practice to employ suitable 3,305,290- Patented Feb. 21, 1967 ice getter materials within the beam tube. Getter is almost frequently used in that portion of the molecular beam device between the oven source and the A magnet. For example, in a molecular beam device employing an atomic cesium beam a thin coating of colloidal graphite has been placed on the internal walls of the beam tube as a getter material. This practice has not been entirely acceptable since after approximately 500 to 1000 hours the graphite becomes saturated with condensed cesium. This rapid exhaustion rate of the thin graphite coating is not desired. The use of thicker carbon collars or cylinders within the beam tube has also not been wholly successful due to the tendency of the cesium vapor to release non-chemical reactive gases from the carbon structure which gases are not readily removable by an ion pump. Thus there has existed a need for a suitable getter material for use for atomic cesium beams which getter material should be an efficient getter having a long life and having a vapor pressure of less than 1X10 mm. of mercury at 50 C.

Accordingly it is a principal object of this invention to provide a superior getter material for cesium. Another object is to provide an improved molecular beam tube device which has a reduced background detector current of atomic cesium. A further object of this invention is to provide an atomic cesium getter for use at elevated temperatures which getter has a vapor pressure of less than 1 10 mm. at 50 C. Still another object of this invention is to provide a molecular beam device employing an atomic cesium beam which permits operation of the device at elevated temperatures without excessive operating beam tube pressures. Furthermore it is an object of the instant invention to provide an improved vacuum envelope for use in molecular beam devices which vacuum envelopes contain a gettering material which efficiently getters cesium at elevated temperatures. Another object of this invention is to provide an improved vacuum envelope having a superior gettering material for atomic cesium which envelope is characterized by an en hanced gettering operational life. Other objects of this invention will be apparent to those skilled in the art and will appear in particular from the following detailed disclosure of the invention when read in conjunction with the attached drawings wherein:

FIG. 1 is a graphical representation showing the vapor pressure of cesium as a function of temperature for liquid cesium and cesium antimonide.

FIG. 2 is a schematic view and a partial cross-sectional view of an oven to A magnet portion of a molecular beam device containing the discovered getter material.

It has now been discovered that antimony is a superior getter material for cesium. Elemental metallic antimony having a melting point of approximately 630 C. reacts with the background cesium flux in the molecular beam tube to form a cesium-antimony-containing compound having a very low vapor pressure and therefore a very low re-effusion rate. The cesium antimonide (Cs Sb) has a vapor pressure of l 10 or less at a temperature of 65 C. Antimony or an antimony-containing compound such as an antimony containing alloy is commonly employed 'by lining the internal wall surfaces of the beam tube so that they react with the cesium background flux. Antimony is a very efficient getter for cesium as can be seen by the quantitative formula wherein, for every atom of antimony employed as a getter three atoms of cesium are gettered or removed from the cesium vapor background flux. The reaction of the antimony with the atomic cesium to form a stable low vapor pressure cesium antimony precedes at room temperature and increases as the temperature increases. Cesium antimonide decomposes into elements at a temperature of about 230 C. Cesium antimonide is well-known as a photoemitter and is commonly used as a photocathode for photocells and photomultipliers. The use of elemental antimony on the internal wall surfaces of a beam tube or within any other evacuated or non-evacuated vessel where cesium atoms are desired to be gettered results in the formation of cesium antimonide such as a cesium antimonide wall surface.

Turning now particularly to FIG. 1 the lower effusion rate of cesium antimonide as compared with liquid cesium can readily be seen. Thus at a temperature of 25 C. the cesium vapor pressure of cesium antimonide is approximately 100 times less than the cesium vapor pressure of liquid cesium at the same temperature. Even at a temperature of 100 C. the vapor pressure of the cesium antimonide is less than the vapor pressure of the liquid cesium at 25 C., the normal operating temperature of a cesium molecular beam device. Further, it should be noticed that at the normal operating temperature of a cesium oven source, that is of about 80 to 125 C., the vapor pressure of the cesium antimonide is 1 10 mm. of mercury or less thereby making antimony a most desirable getter material for use with cesium where a pressure condition of this type is desired. Due to the low reaction rate of the cesium with the antimony at normal storing temperatures e.g. 25 C., and the very low vapor pressure of the cesium antimonide at these temperatures sealing means such as a valve to prevent the cesium vapor from the cesium source from entering the beam tube at storage temperature is optional. Further, the use of the high melting point elemental antimony is advantageous as a getter in that few if any antimony atoms appear into a gaseous state at the normal operating temperatures of the molecular beam devices.

Cesium antimonide is not an ordinary chemical compound and cannot be defined by commonly expressed chemical terminology. It cannot be classified wholly as an ionic solid that is, a compound formed by the ionic bonding of the constituent elements. It cannot be classified as a homopolar solid that is, a compound formed by covalent bonding of the constituent elements. It cannot be classified as a metallic solid that is, an alloy of the constituent elements. Cesium antimonide shows some characteristics of each of these types and is best described as a normal valency intermetallic compound (K. H. Jack and M. M. Wachtel, Proc. Roy. Soc. A239, 46-60 (1957)).

Turning now more particularly to FIG. 2 there is shown therein a portion of a molecular beam device 10 which comprises in combination a cesium source 12 such as a cesium oven source as described in US. Patent 2,991,389 by E. F. Grant et al., issued July 4, 1961, in which said source oven contains highly purified cesium and which source provides atomic cesium to the molecular beam device by heating the cesium to an elevated temperature of from about 50 to 100 C. such as about 65 C. In gaseous flow communication with the cesium oven source there is a gooseneck opening 14 which permits the effusion of the cesium atoms to a collimator means 16. The collimator means is employed to form the atomic cesium beam in a predetermined geometric beam shape and direction. Commonly the collimator means comprises a series of alternating flat strips and corrugated strips of nickel foil to form a plurality of tubular like members of elongated longitudinally disposed channels of preselected cross-sectional area and shape. The collimator means directs the beam of atomic cesium into one end of a vacuum envelope 18 which vacuum envelope comprises a portion of the molecular beam tube and as shown is an elongated longitudinally disposed tubular member, having at the other end thereof an externally disposed A magnet pole piece 20. Disposed within the vacuum envelope 18 and forming the internal wall surface thereof are a series of axially aligned elongated longitudinally disposed cylindrically shaped getter inserts 22a, 22b and 22c containing antimony. These antimony-containing inserts efficiently getter the undesirable atomic cesium background flux in this vacuum envelope and thereby prevents undesired detector background current.

With an oven operating temperature of approximately 65 C. there normally exists a temperature gradient within the vacuum envelope ranging from a high of 65 C. at the collimator end to about 25-30 C. at the A magnet pole piece end of the envelope. It is within this particular temperature gradient area that the need for a getter material is particularly great since the elevated temperature creates an undesirably high cesium background flux. This evacuated vacuum envelope area constitutes that area where there exists a portion of the beam not used for resonance purposes. Of course the entire evacuated molecular beam tube between the oven source and the detector may also be lined with an antimony getter means as shown. Thus where the entire molecular beam tube is operated at an elevated environmental temperature creating an undesirable cesium background flux it may be desirable to line the entire molecular beam tube with the antimony getter or only portions of the molecular beam tube. Normally however that area of the molecular beam tube, not shown, ranging between the A magnet, the C magnet, the B magnet and the microwave cavity and to the cesium detector does not require the use of a getter.

The antimony getter inserts 22a, 22b and 220 employed as shown are vacuum cast from elemental antimony metal by conventional vacuum casting techniques. These inserts commonly have a thickness of 0.005 inch or more with cylinders of from 0.06 to 0.12 inch usually commercially employed to provide inserts of desired mechanical strength and rigidity. Antimony is characterized by being a grey colored hard brittle metal which in reaction with cesium progressively turns to a bluish purple color. The use of antimony-containing compounds which compounds contain antimony in ionic or covalent bonding states is not particularly advantageous since the antimony must be transformed from these states into the elemental antimony state before reaction with the cesium is accomplished. The presently preferred and efficient method of providing gettering means is to vacuum cast a cylinder of pure metallic antimony to provide a relatively homogeneous interior wall surface of antimony. Where desired, and especially where only a small amount of gettering action is needed the antimony or antimony-containing compound such as an antimony-containing alloy may be vacuum deposited by common vacuum deposition means on other casting inserts such as carbon or on all or a portion of the internal wall surface of the molecular beam tube. Furthermore, the antimony getter can also be employed within an evacuated vessel wherein the powdered state will not interfere with the operation of the particular device wherein it is employed.

It is within the scope of the present invention that alloys containing antimony as a major or minor constituent may also be employed as cesium gettering means. Thus the tensile strength of antimony can be improved by alloying with other metals. Thus the cylindrical getter inserts can be composed of an antimony-containing alloy. Additionally, antimony powder can be compacted and sintered into the desired form by typical powder metallurgical methods to form a relatively porous antimony or antimony-containing alloy getter insert. Additionally, elemental antimony may also be exposed within other inorganic materials such as clay, silica, alumina, carbon, glass or other ceramic or vitreous like materials capable of being formed in a coherent unitary body for formation in the desired shape. Also the antimony may be disposed in organic material such as thermoplastic polyolefins like polyethylene and polypropylene or may be placed in elevated temperature resistant plastics such as in thermosetting resins capable of being molded and heat cured across length into a desired form. Suitable resins and plastics will thus include the aldehyde-phenol resins like phenol formaldehyde resins, urea-aldehyde resins like urea-formaldehyde resins, polyesters, polycarbonates, polyethers like ureathanes, polyamides like nylon, polyvinyl compounds like polystyrene and alike. However, it should be recognized with these other than pure antimony inserts or other supporting antimony means that only those particular internal wall sites containing antimony will serve a gettering action. Thus the employment of an antimonycontaining alloy as the internal wall surface of the vacuum envelope will not provide the same degree of gettering action as a pure antimony Wall surface. The selection of the particular state of the antimony and the means of disposing the antimony in a gettering position depends upon the particular area in which the antimony is to be used, the temperature of the area as well as the vacuum conditions desired.

There has thus been described an efiicient method and apparatus for a gettering atomic cesium with elemental antimony. The use of antimony as a getter accomplishes the objects set forth above as made apparent from the preceding description. It is of course recognized that certain changes may be made in the above method of disposing the antimony getter without departing from the scope of this invention and it is therefore intended by all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative only and not in a limited sense.

Having described the invention what is claimed is:

1. A method of gettering cesium which method comprises contacting atomic cesium with a gettering amount of an antimony-containing material.

2. A method as defined in claim 1 in which said contacting occurs at sub-atmospheric pressures.

3. A method as defined in claim 1 in which said contacting occurs at a temperature of between 25 and 200 C.

4. A method as defined in claim 1 wherein the antimony-containing material is elemental antimony metal.

5. A method as defined in claim 1 wherein the antimony-containing material comprises an elemental antimony containing metal alloy.

6. A method as defined in claim 1 wherein the gettering amount of the antimony-containing material is sufil cient to maintain a cesium atmospher of less than 1x10 mm. of mercury.

7. A method as defined in claim 1 wherein the antimony-containing material is in solid film form.

8. A method of gettering cesium from a vessel at subatmospheric pressures which method comprises: contacting the atomic cesium vapor at a temperature less than the decomposition temperature of cesium antimonide with a gettering amount of an elemental antimony-containing compound.

9. A method as defined in claim 8 wherein the antimony-containing compound is elemental antimony disposed on the internal wall surfaces of the vessel.

. 10. A method of controlling atomic cesium background fluX in a cesium molecular beam device to inhibit undesired background current in a cesium detector which device includes a heated cesium source, collimator means, a molecular beam tube, A, B, and C external magnet means, oscillating field means, and a cesium detector which method comprises:

placing on the internal wall surfaces of at least a portion of the molecular beam tube a gettering film amount of an elemental antimony-containing material, and,

contacting said antimony-containing material with the background cesium flux not employed for resonance purpose to form cesium antimonide, thereby reducing the cesium flux background current in the cesium detector.

11. A method as defined in claim 10 wherein the internal wall surfaces of that portion of the molecular beam device between the collimator means and the A magnet is lined with an elemental antimony-containing metal.

12. A method as defined in claim 11 wherein the internal wall surfaces are lined with elemental antimony having a film thickness of at least 0.005 inch.

13. A method as defined in claim 10 which includes vacuum casting forms of predetermined geometry and of elemental antimony said forms being capable of being inserted within the molecular beam tube to provide an internal wall surface of elemental antimoney.

References Cited by the Examiner UNITED STATES PATENTS 2,898,501 8/1959 Wadia et a1. 313174 2,899,257 8/1959 Ledercr 3l625 3,062,980 11/1962 Bawdekar 313-173 3,131,983 5/1964 Harries 3l625 RICHARD H. EANES, JR., Primary Examiner. GEORGE N. WESTBY, Examiner.

S. D. SCHLOSSER, Assistant Examiner. 

1. A METHOD OF GETTERING CESIUM WHICH METHOD COMPRISES CONTACTING ATOMIC CESIUM WITH A GETTERING AMOUNT OF AN ANTIMONY-CONTAINING MATERIAL. 